(1) Field of the Invention
The present invention relates to the field of regulating the operation of each of the engines in a power plant of a multi-engined rotorcraft. Said power plant comprises in particular fuel-burning main engines, in particular turboshaft engines, that conventionally deliver to the rotorcraft the mechanical power needed at least for driving one or more of the rotors of the rotorcraft.
The present invention lies more specifically in the context of a failure of at least one of said main engines of the rotorcraft that drive rotation of at least one main rotor of the rotorcraft at a variable setpoint speed.
(2) Description of Related Art
The main rotor of a rotorcraft typically provides the rotorcraft at least with lift, and possibly also in the specific example of a helicopter, with propulsion and/or the ability to change attitude in flight. An anti-torque rotor of the rotorcraft typically serves to stabilize and guide the rotorcraft in yaw and is commonly formed by a tail rotor or by at least one propulsive propeller in a rotorcraft having high forward speeds.
Conventionally, the operation of each of the main engines of a rotorcraft is placed under the control of a full authority digital engine control (FADEC). The FADEC controls the metering of fuel to the main engines as a function of the mechanical power requirements of the rotorcraft, and in particular as a function of the mechanical power requirements for driving the main rotor at a required speed of rotation as identified by a speed setpoint, referred to as the NR setpoint.
The mechanical power needs of a rotorcraft are potentially identified by a flight control unit, such as an automatic flight control system (AFCS). For example, the mechanical power needed by the main rotor may be identified by the on-board instrumentation of the rotorcraft on the basis of a resistive torque that the main rotor opposes against being driven by the power plant.
In this context, the current operating rating of the power plant is controlled by the regulator unit depending on various regulation ratings that are identified as a function of a nominal regulating rating commonly referred to as the all engines operative (AEO) rating.
The regulation of the operating rating of the power plant serves to avoid damaging one or more main engines as a result of making excessive use of the capacities of the power plant for delivering the mechanical power required by the rotorcraft. Several limit criteria are taken into account by the regulator unit in order to avoid such excessive use being made of the capacities of the power plant. Such limit criteria include, by way of example:
a criterion for limiting the speed of the gas generator(s) of the engine(s);
a criterion for limiting the temperature of the free turbine(s) of the engine(s) as driven by the gas generator(s); and
a criterion for limiting the torque from the free turbine and/or at the inlet of a main mechanical power transmission gearbox, which gearbox has at least the rotor(s) engaged therewith in order to be driven in rotation.
In addition to the AEO rating, various specific operating ratings for the power plant are commonly defined as a function of stages of flight of the rotorcraft. These specific regulation ratings of the AEO rating include in particular:
a maximum continuous power (MCP) rating defining the maximum rating that is authorized for continuous use of the engine(s) depending on the constraints imposed by said limit criteria. Such an MCP rating is commonly used while the rotorcraft is in cruising flight;
a maximum takeoff power (TOP) rating defining the maximum rating authorized for the engine(s) for use over a predetermined duration, which by way of example is about 30 minutes (min), and which is defined as sufficing for enabling the rotorcraft to take off. Such a maximum TOP rating is also commonly used while a rotorcraft is hovering;
a maximum transient power (MTP) rating defining the maximum rating that is authorized for the engine(s) for use during a transient stage of changing the forward speed of the rotorcraft, in particular while the rotorcraft is accelerating. The MTP rating is used for a short duration, and by way of example it may be of the order of about ten seconds or one minute.
In this context, there arises the problem of a failure of one of the main engines of a twin-engined rotorcraft or of a plurality of the main engines of a rotorcraft having more than two main engines. Under such circumstances, it can happen that only one main engine of the rotorcraft remains operational and needs, on its own, to deliver all of the mechanical power required by the rotorcraft.
That is why specific ratings have been developed for regulating the operation of main engines in the event of a main engine failing, and they are commonly referred to as one engine inoperative (OEI) ratings.
OEI ratings are applied to regulate the operation of a main engine that is delivering on its own the mechanical power needed by the rotorcraft in flight in the event of at least one other main engine of a multi-engined rotorcraft failing. OEI ratings are typically defined for specific stages of flight in compliance with a given level of mechanical power to be delivered for a given period by the main engine while ensuring that the engine is not degraded beyond an acceptable degradation threshold. Various OEI ratings may potentially be applied by the regulator unit, either automatically (by an automatic controller) or on request of the pilot of the rotorcraft in compliance with the flight manual. The following OEI ratings are commonly defined:
a very short duration OEI rating in which each still-operational main engine(s) can individually be used at a contingency rating for a short duration of the order of 30 seconds (s);
a short duration OEI rating in which each still-operational main engine is individually usable at a contingency rating for a short duration of the order of 2 min to 3 min;
a long duration OEI rating during which each still-operational main engine is individually suitable for being used a defined maximum contingency rating for a duration that is long, and potentially unlimited.
Furthermore, a rotorcraft is conventionally fitted with at least one display unit for the purpose of using a screen to give the pilot information about the flight state of the rotorcraft and in particular information about the operating state of each engine.
Such a display unit may for example be of the type commonly known as a flight limit indicator (FLI).
On the basis of data about the operating state of each main engine, and taking account in particular at least of said limit criteria and of the current regulation rating of each main engine, a screen displays mechanical power margin information, referred to below as the “authorized” margin, that can be made use of by the pilot without damaging the power plant. The pilot then generates flight controls while ensuring that the power plant is not called on to deliver mechanical power exceeding said authorized margin.
Concerning the NR setpoint, it is defined so as to obtain a speed of rotation for the main rotor, referred to below as the NR speed, and that is conventionally predefined as being substantially constant.
In that traditional context, the NR speed varies at most over a narrow range of speed variation of the order of 5% of a nominal speed, while nevertheless not exceeding a rate of variation of about 1% per second. The impact of such limited variation of the NR speed is negligible for the variation in the mechanical power than the main engines of the rotorcraft need to deliver in order to drive the main rotor.
Specifically, a failure of one of the main engines of the rotorcraft leads to a sudden loss of mechanical power that can be delivered by the power plant. As a result of such a sudden loss of mechanical power, the NR speed drops. Nevertheless, at the instant of one of the main engines of the rotorcraft failing, the current NR speed is substantially equal to the nominal speed and is still sufficient to enable the pilot to have easy control over the attitude of the rotorcraft.
Furthermore, depending on the equipment of the rotorcraft, an autopilot may potentially be used to act quickly to re-establish safe flight conditions for the rotorcraft in the event of one of the main engines failing, with the autopilot generating automatic flight control signals that serve in particular to modify the current collective pitch of the blades of the main rotor in order to ensure stabilized lift of the rotorcraft.
In general terms, such an autopilot that may potentially be fitted to a rotorcraft is a member for automatically assisting in piloting that typically generates automatic flight control signals for causing collective and/or cyclic variation in the pitch of the blades of the main rotor, and possibly also collective variation in the pitch of the blades of said at least one auxiliary rotor, e.g. a tail rotor.
Conventionally, the automatic flight control signals are generated by the autopilot in application of flight setpoints previously sent to the autopilot by the human pilot of the rotorcraft by means of various human-actuatable control members, such as in particular manual flight control members.
When the autopilot is on, the flight setpoints are processed by the autopilot in order to generate automatic flight control signals in compliance with applying various predefined modes of operation of the autopilot that are implemented on being selected by the human pilot.
Such modes of operation of the autopilot include in particular at least a basic mode providing automatic assistance for stabilizing the rotorcraft in flight, and/or higher modes of operation that provide automatic guidance of the rotorcraft.
In this context, the autopilot applies a given operating mode in compliance with the available mechanical power that can be delivered by the power plant depending on its operating state as identified by said limit criteria and depending on the current regulation rating being applied by the regulator unit.
Naturally, in this context, the autopilot has available information that is delivered by the on-board instrumentation about the operating state of the power plant as identified by the limit criteria. The autopilot generates automatic flight control signals while taking account of said authorized margin, including in the event of at least one of the main engines failing, in order to avoid degrading the main engines.
Nevertheless, technical trends in the field of rotorcraft are favoring driving the main rotor at a controlled NR speed that is variable relative to the nominal speed, depending on the flight conditions of the rotorcraft. Such significant variation in the NR speed at which the main rotor is driven in rotation can be used, by way of example, for the purpose of reducing the sound nuisance of the rotorcraft and/or for improving its performance during certain stages of flight, or indeed for adapting the NR speed as a function of weather conditions and/or as a function of the situation in which the rotorcraft is placed.
By way of indication, in this context of recent techniques involving controlling variation in the NR speed, the speed of the rotor speed may be controlled to vary in the range 5% to 20% of the nominal speed, and possibly even more, with changing techniques. By way of indication, the NR speed is presently commonly controlled to vary over a range of values that may lie potentially from 90% to 107% of the nominal speed.
In this context, reference may be made to Document XP0000 Schaefer 1990 “Enhanced energy maneuverability for attack helicopters using continuous variable rotor speed control” (C. G. Schaefer Jr.; F. H. Lutze Jr.); 47th Forum American Helicopter Society 1991; pp. 1293-1303. According to that document, the performance of a rotorcraft in a combat situation is improved by varying the speed at which the main rotor is driven in rotation depending on variation in the air speed of the rotorcraft.
Reference may also be made by way of example to Document U.S. Pat. No. 6,198,991, which proposes reducing the sound nuisance generated by a rotorcraft close to a landing point by varying the speed of rotation of the main rotor.
In this context, reference may also be made by way of example to Document US 2007/118254, which proposes varying the speed of rotation of the main rotor of a rotorcraft between two values referred to as “high” and “low”, under predefined threshold conditions for the values of various parameters associated with previously identified flight conditions of the rotorcraft.
Also by way of example, reference may also be made on this topic to Document WO 2010/143051, which proposes varying the speed of rotation of a main rotor of a rotorcraft in compliance with a previously established map depending on various flight conditions of the rotorcraft.
There then arises the problem of ways of acting on the behavior of the rotorcraft in the event of a failure of one of its main engines, given that the main rotor may potentially be being driven at an NR speed that is low compared with its nominal speed, which speed may be at least 7% less than the nominal speed. Under such circumstances, it is much more difficult for the pilot to bring the drive of the main rotor back to an NR speed that is in compliance with the NR setpoint.
Consequently, it appears to be appropriate to provide the pilot of a multi-engined rotorcraft with assistance in piloting in order to act, in the event of one of the main engines failing, for the purpose of bringing the drive of the main rotor back quickly to an NR speed that ensures safe progress for the rotorcraft in the context of it being possible that the main rotor was being driven at an NR speed that is low compared with the nominal speed at the instant when said one of the main engines fails.
The technological background of the invention includes the situation of a single-engined rotorcraft where automatic assistance is provided for the pilot of the rotorcraft in order to place the main rotor in autorotation in the event of the main engine failing.
Such assistance is provided by an automatic device that acts in the event of the main engine failing to generate flight control signals for the purpose of modifying the attitude of the rotorcraft in vertical, pitching, roll, and/or yaw terms, in order to counterbalance the unfavorable aerodynamic effects that occur immediately after a failure of the main engine.
Reference may be made for example on this topic to the following documents: GB 2 192 163; US 2005/135930; and US 2013/0221153.